Programming a Multiplayer FPS in DirectX/Programming a Multiplayer FPS in DirectX (Companion CD).iso/Source/Chapter 12/Engine/CollisionDetection.h

C/C++ Source or Header | 2004-10-01 | 24.4 KB | 592 lines

//-----------------------------------------------------------------------------
// Provides basic collision detection in a 3D environment that includes both
// static geometry and dynamic objects.
//
// Note: This uses an adaptation of the algorithm from the Improved Collision
//       Detection and Response article written by Kasper Fauerby.
//
// Programming a Multiplayer First Person Shooter in DirectX
// Copyright (c) 2004 Vaughan Young
//-----------------------------------------------------------------------------
#ifndef COLLISION_H
#define COLLISION_H

//-----------------------------------------------------------------------------
// Collision Data Structure
//-----------------------------------------------------------------------------
struct CollisionData
{
    float scale; // Scale used by thge calling scene manager.
    float elapsed; // Elapsed time for the current frame.
    unsigned long frameStamp; // Current frame stamp according to the scene manager.

    SceneObject *object; // Pointer to the object to perform collision detection with.

    D3DXVECTOR3 translation; // Translation in ellipsoid space.
    D3DXVECTOR3 velocity; // Velocity in ellipsoid space.
    D3DXVECTOR3 normalizedVelocity; // Normalized velocity in ellipsoid space.

    D3DXVECTOR3 gravity; // Gravity vector, which will be converted to ellipsoid space.

    bool collisionFound; // Indicates if a collision has been found.
    float distance; // Distance to the point of collision.
    D3DXVECTOR3 intersection; // Actual intersection point where the collision occured.
};

//-----------------------------------------------------------------------------
// Returns the lowest root of a quadratic equation.
//-----------------------------------------------------------------------------
inline float GetLowestRoot( float a, float b, float c, float max )
{
    // Calculate the determinant, then get the square root of it if it's valid.
    float determinant = b * b - a * c;
    if( determinant < 0.0f )
        return 0.0f;
    determinant = (float)sqrt( determinant );

    // Calculate the first root and ensure it is within the bounds.
    float root1 = ( b + determinant ) / a;
    if( root1 <= 0.0f || root1 > max )
        root1 = max + 1.0f;

    // Calculate the second root and ensure it is within the bounds.
    float root2 = ( b - determinant ) / a;
    if( root2 <= 0.0f || root2 > max )
        root2 = max + 1.0f;

    // Get the lowest of the two roots.
    float root = min( root1, root2 );

    // Ensure the root is valid.
    if( root == max + 1.0f )
        return 0.0f;

    return root;
}

//-----------------------------------------------------------------------------
// Checks a single face for intersection.
//-----------------------------------------------------------------------------
inline void CheckFace( CollisionData *data, D3DXVECTOR3 vertex0, D3DXVECTOR3 vertex1, D3DXVECTOR3 vertex2 )
{
    // Create a plane from the face's vertices.
    D3DXPLANE plane;
    D3DXPlaneFromPoints( &plane, &vertex0, &vertex1, &vertex2 );

    // Get the angle between the plane's normal and the velocity vector.
    float angle = D3DXPlaneDotNormal( &plane, &data->normalizedVelocity );

    // Ensure the plane is facing the velocity vector (i.e. ignore back faces).
    if( angle > 0.0f )
        return;

    // Get the plane's normal vector.
    D3DXVECTOR3 planeNormal;
    D3DXVec3Cross( &planeNormal, &( vertex0 - vertex1 ), &( vertex1 - vertex2 ) );
    D3DXVec3Normalize( &planeNormal, &planeNormal );

    // Calculate the signed distance from sphere's translation to plane.
    float signedPlaneDistance = D3DXVec3Dot( &data->translation, &planeNormal ) + plane.d;

    // Get interval of plane intersection
    float time0, time1;
    bool embedded = false;

    // Cache this as we're going to use it a few times below
    float normalDotVelocity = D3DXVec3Dot( &planeNormal, &data->velocity );

    // Check if the sphere is travelling parallel to the plane.
    if( normalDotVelocity == 0.0f )
    {
        // If the sphere is not embedded in the plane, then it cannot collide.
        if( fabs( signedPlaneDistance ) >= 1.0f )
            return;
        else
        {
            // The sphere is embedded in plane, therefore it will collide
            // for the entire time frame.
            embedded = true;
            time0 = 0.0f;
            time1 = 1.0f;
        }
    }
    else
    {
        // Calculate the time frame of intersection.
        time0 = ( -1.0f - signedPlaneDistance ) / normalDotVelocity;
        time1 = ( 1.0f - signedPlaneDistance ) / normalDotVelocity;

        // Ensure time0 is less than time1.
        if( time0 > time1 )
        {
            float swap = time1;
            time1 = time0;
            time0 = swap;
        }

        // If the intersection time frame is out of range, then it cannot collide.
        if( time0 > 1.0f || time1 < 0.0f )
            return;

        // Normalize the time frame.
        if( time0 < 0.0f ) time0 = 0.0f;
        if( time1 < 0.0f ) time1 = 0.0f;
        if( time0 > 1.0f ) time0 = 1.0f;
        if( time1 > 1.0f ) time1 = 1.0f;
    }

    // Variables used for tracking if an intersection occured, where it happened, and when.
    bool intersectFound = false;
    D3DXVECTOR3 intersection;
    float intersectTime = 1.0f;

    // Check if the sphere is embedded in the plane.
    if( embedded == false )
    {
        // Get the plane intersection point at time0.
        D3DXVECTOR3 planeIntersectionPoint = ( data->translation - planeNormal ) + data->velocity * time0;

        // Get the vectors of two of the face's edges.
        D3DXVECTOR3 edge0 = vertex1 - vertex0;
        D3DXVECTOR3 edge1 = vertex2 - vertex0;

        // Get the angles of the edges and combine them.
        float angle0 = D3DXVec3Dot( &edge0, &edge0 );
        float angle1 = D3DXVec3Dot( &edge0, &edge1 );
        float angle2 = D3DXVec3Dot( &edge1, &edge1 );
        float combined = ( angle0 * angle2 ) - ( angle1 * angle1 );

        // Get the split angles between the two edges.
        D3DXVECTOR3 split = planeIntersectionPoint - vertex0;
        float splitAngle0 = D3DXVec3Dot( &split, &edge0 );
        float splitAngle1 = D3DXVec3Dot( &split, &edge1 );

        float x = ( splitAngle0 * angle2 ) - ( splitAngle1 * angle1 );
        float y = ( splitAngle1 * angle0 ) - ( splitAngle0 * angle1 );
        float z = x + y - combined;

        // Take the bitwise AND of z and the complement of the inclusive OR of x and y,
        // then bitwise AND the result with 0x80000000 and return it. A bitwise result
        // of zero equals false, while any other value equals true.
        if( ( ( (unsigned int&)z & ~( (unsigned int&)x | (unsigned int&)y ) ) & 0x80000000 ) != 0 )
        {
            intersectFound = true;
            intersection = planeIntersectionPoint;
            intersectTime = time0;
        }
    }

    // Check if a collision has been found yet.
    if( intersectFound == false )
    {
        // Get the squared length of the velocity vector.
        float squaredVelocityLength = D3DXVec3LengthSq( &data->velocity );

        // A quadratic equation has to be solved for each vertex and edge in the face.
        // The following variables are used to build each quadratic equation.
        float a, b, c;

        // Used for storing the result of each quadratic equation.
        float newTime;

        // First check againts the vertices.
        a = squaredVelocityLength;

        // Check vertex 0.
        b = 2.0f * D3DXVec3Dot( &data->velocity, &( data->translation - vertex0 ) );
        c = D3DXVec3LengthSq( &( vertex0 - data->translation ) ) - 1.0f;
        if( newTime = GetLowestRoot( a, b, c, intersectTime ) > 0.0f )
        {
            intersectFound = true;
            intersection = vertex0;
            intersectTime = newTime;
        }

        // Check vertex 1.
        b = 2.0f * D3DXVec3Dot( &data->velocity, &( data->translation - vertex1 ) );
        c = D3DXVec3LengthSq( &( vertex1 - data->translation ) ) - 1.0f;
        if( newTime = GetLowestRoot( a, b, c, intersectTime ) > 0.0f )
        {
            intersectFound = true;
            intersection = vertex1;
            intersectTime = newTime;
        }

        // Check vertex 2.
        b = 2.0f * D3DXVec3Dot( &data->velocity, &( data->translation - vertex2 ) );
        c = D3DXVec3LengthSq( &( vertex2 - data->translation ) ) - 1.0f;
        if( newTime = GetLowestRoot( a, b, c, intersectTime ) > 0.0f )
        {
            intersectFound = true;
            intersection = vertex2;
            intersectTime = newTime;
        }

        // Check the edge from vertex0 to vertex1.
        D3DXVECTOR3 edge = vertex1 - vertex0;
        D3DXVECTOR3 vectorSphereVertex = vertex0 - data->translation;
        float squaredEdgeLength = D3DXVec3LengthSq( &edge );
        float angleEdgeVelocity = D3DXVec3Dot( &edge, &data->velocity );
        float angleEdgeSphereVertex = D3DXVec3Dot( &edge, &vectorSphereVertex );

        // Get the parameters for the quadratic equation.
        a = squaredEdgeLength * -squaredVelocityLength + angleEdgeVelocity * angleEdgeVelocity;
        b = squaredEdgeLength * ( 2.0f * D3DXVec3Dot( &data->velocity, &vectorSphereVertex ) ) - 2.0f * angleEdgeVelocity * angleEdgeSphereVertex;
        c = squaredEdgeLength * ( 1.0f - D3DXVec3LengthSq( &vectorSphereVertex ) ) + angleEdgeSphereVertex * angleEdgeSphereVertex;

        // Check if the sphere intersects the edge.
        if( newTime = GetLowestRoot( a, b, c, intersectTime ) > 0.0f )
        {
            // Ensure the intersection occured within the edges bounds.
            float point = ( angleEdgeVelocity * newTime - angleEdgeSphereVertex ) / squaredEdgeLength;
            if( point >= 0.0f && point <= 1.0f )
            {
                intersectFound = true;
                intersection = vertex0 + edge * point;
                intersectTime = newTime;
            }
        }

        // Check the edge from vertex1 to vertex2.
        edge = vertex2 - vertex1;
        vectorSphereVertex = vertex1 - data->translation;
        squaredEdgeLength = D3DXVec3LengthSq( &edge );
        angleEdgeVelocity = D3DXVec3Dot( &edge, &data->velocity );
        angleEdgeSphereVertex = D3DXVec3Dot( &edge, &vectorSphereVertex );

        // Get the parameters for the quadratic equation.
        a = squaredEdgeLength * -squaredVelocityLength + angleEdgeVelocity * angleEdgeVelocity;
        b = squaredEdgeLength * ( 2.0f * D3DXVec3Dot( &data->velocity, &vectorSphereVertex ) ) - 2.0f * angleEdgeVelocity * angleEdgeSphereVertex;
        c = squaredEdgeLength * ( 1.0f - D3DXVec3LengthSq( &vectorSphereVertex ) ) + angleEdgeSphereVertex * angleEdgeSphereVertex;

        // Check if the sphere intersects the edge.
        if( newTime = GetLowestRoot( a, b, c, intersectTime ) > 0.0f )
        {
            // Ensure the intersection occured within the edges bounds.
            float point = ( angleEdgeVelocity * newTime - angleEdgeSphereVertex ) / squaredEdgeLength;
            if( point >= 0.0f && point <= 1.0f )
            {
                intersectFound = true;
                intersection = vertex1 + edge * point;
                intersectTime = newTime;
            }
        }

        // Check the edge from vertex2 to vertex0.
        edge = vertex0 - vertex2;
        vectorSphereVertex = vertex2 - data->translation;
        squaredEdgeLength = D3DXVec3LengthSq( &edge );
        angleEdgeVelocity = D3DXVec3Dot( &edge, &data->velocity );
        angleEdgeSphereVertex = D3DXVec3Dot( &edge, &vectorSphereVertex );

        // Get the parameters for the quadratic equation.
        a = squaredEdgeLength * -squaredVelocityLength + angleEdgeVelocity * angleEdgeVelocity;
        b = squaredEdgeLength * ( 2.0f * D3DXVec3Dot( &data->velocity, &vectorSphereVertex ) ) - 2.0f * angleEdgeVelocity * angleEdgeSphereVertex;
        c = squaredEdgeLength * ( 1.0f - D3DXVec3LengthSq( &vectorSphereVertex ) ) + angleEdgeSphereVertex * angleEdgeSphereVertex;

        // Check if the sphere intersects the edge.
        if( newTime = GetLowestRoot( a, b, c, intersectTime ) > 0.0f )
        {
            // Ensure the intersection occured within the edges bounds.
            float point = ( angleEdgeVelocity * newTime - angleEdgeSphereVertex ) / squaredEdgeLength;
            if( point >= 0.0f && point <= 1.0f )
            {
                intersectFound = true;
                intersection = vertex2 + edge * point;
                intersectTime = newTime;
            }
        }
    }

    // Check if an intersection occured.
    if( intersectFound == true )
    {
        // Get the distance to the collision (i.e. time along the velocity vector).
        float collisionDistance = intersectTime * D3DXVec3Length( &data->velocity );

        // Store the collision details, if necessary.
        if( data->collisionFound == false || collisionDistance < data->distance )
        {
            data->distance = collisionDistance;
            data->intersection = intersection;
            data->collisionFound = true;
        }
    }
}

//-----------------------------------------------------------------------------
// Perfrom collision detection between the give object and the scene.
//-----------------------------------------------------------------------------
inline void CollideWithScene( CollisionData *data, Vertex *vertices, SceneFace *faces, unsigned long totalFaces, LinkedList< SceneObject > *objects, unsigned long recursion = 5 )
{
    // Calculate the epsilon distance (taking scale into account).
    // The epsilon distance is a very short distance that is considered negligable.
    float epsilon = 0.5f * data->scale;

    // Indicate that a collision has not been found.
    data->collisionFound = false;

    // Get the normalized velocity vector.
    D3DXVec3Normalize( &data->normalizedVelocity, &data->velocity );

    // Go through all of the faces.
    D3DXVECTOR3 vertex0, vertex1, vertex2;
    for( unsigned long f = 0; f < totalFaces; f++ )
    {
        // Skip this face if its material is set to ignore rays.
        if( faces[f].renderCache->GetMaterial()->GetIgnoreRay() == true )
            continue;

        // Get a copy of this face's vertices in ellipsoid space.
        vertex0.x = vertices[faces[f].vertex0].translation.x / data->object->GetEllipsoidRadius().x;
        vertex0.y = vertices[faces[f].vertex0].translation.y / data->object->GetEllipsoidRadius().y;
        vertex0.z = vertices[faces[f].vertex0].translation.z / data->object->GetEllipsoidRadius().z;
        vertex1.x = vertices[faces[f].vertex1].translation.x / data->object->GetEllipsoidRadius().x;
        vertex1.y = vertices[faces[f].vertex1].translation.y / data->object->GetEllipsoidRadius().y;
        vertex1.z = vertices[faces[f].vertex1].translation.z / data->object->GetEllipsoidRadius().z;
        vertex2.x = vertices[faces[f].vertex2].translation.x / data->object->GetEllipsoidRadius().x;
        vertex2.y = vertices[faces[f].vertex2].translation.y / data->object->GetEllipsoidRadius().y;
        vertex2.z = vertices[faces[f].vertex2].translation.z / data->object->GetEllipsoidRadius().z;

        // Check for collision with this face.
        CheckFace( data, vertex0, vertex1, vertex2 );
    }

    // Create a list of hit ghost objects and a list of the distances to them.
    LinkedList< SceneObject > *ghostHits = new LinkedList< SceneObject >;
    LinkedList< float > *ghostDistances = new LinkedList< float >;

    // Variables used for the following object collision check.
    D3DXVECTOR3 translation, velocity, vectorColliderObject, vectorObjectCollider, vectorObjectRadius;
    float distToCollision, colliderRadius, objectRadius;

    // Go through the list of objects.
    SceneObject *hitObject = NULL;
    SceneObject *nextObject = objects->GetFirst();
    while( nextObject != NULL )
    {
        // Skip this object if it is the collider. It can't check against itself.
        if( nextObject != data->object )
        {
            // Get the translation and velocity of this object in ellipsoid space.
            translation.x = nextObject->GetTranslation().x / data->object->GetEllipsoidRadius().x;
            translation.y = nextObject->GetTranslation().y / data->object->GetEllipsoidRadius().y;
            translation.z = nextObject->GetTranslation().z / data->object->GetEllipsoidRadius().z;
            velocity.x = nextObject->GetVelocity().x / data->object->GetEllipsoidRadius().x;
            velocity.y = nextObject->GetVelocity().y / data->object->GetEllipsoidRadius().y;
            velocity.z = nextObject->GetVelocity().z / data->object->GetEllipsoidRadius().z;
            velocity *= data->elapsed;

            // Get the normalized vectors from the collider to this object and vice versa.
            D3DXVec3Normalize( &vectorColliderObject, &( translation - data->translation ) );
            D3DXVec3Normalize( &vectorObjectCollider, &( data->translation - translation ) );

            // Calculate the radius of each ellipsoid in the direction to the other.
            colliderRadius = D3DXVec3Length( &vectorColliderObject );
            vectorObjectRadius.x = vectorObjectCollider.x * nextObject->GetEllipsoidRadius().x / data->object->GetEllipsoidRadius().x;
            vectorObjectRadius.y = vectorObjectCollider.y * nextObject->GetEllipsoidRadius().y / data->object->GetEllipsoidRadius().y;
            vectorObjectRadius.z = vectorObjectCollider.z * nextObject->GetEllipsoidRadius().z / data->object->GetEllipsoidRadius().z;
            objectRadius = D3DXVec3Length( &vectorObjectRadius );

            // Check for collision between the two spheres.
            if( IsSphereCollidingWithSphere( &distToCollision, data->translation, translation, velocity - data->velocity, colliderRadius + objectRadius ) == true )
            {
                // Check if the hit object is a ghost.
                if( nextObject->GetGhost() == true )
                {
                    // If both object's are allowed to register collisions, then store a pointer to the hit object and the distance to hit it.
                    if( nextObject->GetIgnoreCollisions() == false && data->object->GetIgnoreCollisions() == false )
                    {
                        ghostHits->Add( nextObject );
                        ghostDistances->Add( &distToCollision );
                    }
                }
                else
                {
                    // Store the collision details, if necessary.
                    if( data->collisionFound == false || distToCollision < data->distance )
                    {
                        data->distance = distToCollision;
                        data->intersection = data->normalizedVelocity * distToCollision;
                        data->collisionFound = true;

                        // Store a pointer to the hit object.
                        hitObject = nextObject;
                    }
                }
            }
        }

        // Go to the next object.
        nextObject = objects->GetNext( nextObject );
    }

    // Iterate through the list of hit ghost objects and their collision distances.
    ghostHits->Iterate( true );
    ghostDistances->Iterate( true );
    while( ghostHits->Iterate() != NULL && ghostDistances->Iterate() != NULL )
    {
        // If the distance to hit the ghost object is less than the distance to the closets real collision, then the ghost object has been hit.
        if( *ghostDistances->GetCurrent() < data->distance )
        {
            // Register the collision between both objects.
            ghostHits->GetCurrent()->CollisionOccurred( data->object, data->frameStamp );
            data->object->CollisionOccurred( ghostHits->GetCurrent(), data->frameStamp );
        }
    }

    // Destroy the ghost hits and distances lists.
    ghostHits->ClearPointers();
    SAFE_DELETE( ghostHits );
    ghostDistances->ClearPointers();
    SAFE_DELETE( ghostDistances );

    // If no collision occured, then just move the full velocity vector.
    if( data->collisionFound == false )
    {
        data->translation = data->translation + data->velocity;
        return;
    }

    // Calculate the destination (i.e. the point the object was trying to get to).
    D3DXVECTOR3 destination = data->translation + data->velocity;

    // The new translation will be the point where the object actually ends up.
    D3DXVECTOR3 newTranslation = data->translation;

    // Ignore the movement if the object is already very close to its destination.
    if( data->distance >= epsilon )
    {
        // Calculate the new velocity required to move the distance.
        D3DXVECTOR3 newVelocity = data->normalizedVelocity * ( data->distance - epsilon );

        // Find the new translation.
        newTranslation = data->translation + newVelocity;

        // Adjust the polygon intersection point to taking into account that fact that
        // the object does not move right up to the actual intersection point.
        D3DXVec3Normalize( &newVelocity, &newVelocity );
        data->intersection = data->intersection - newVelocity * epsilon;
    }

    // Check if the collision occured with an object.
    if( hitObject != NULL )
    {
        // Set the new translation of the object.
        data->translation = newTranslation;

        // Calculate and apply a push velocity so objects can push one another.
        D3DXVECTOR3 push = ( hitObject->GetVelocity() + data->object->GetVelocity() ) / 10.0f;
        hitObject->SetVelocity( push );
        data->object->SetVelocity( push );

        // Register the collision between both objects, if thay are allowed.
        if( hitObject->GetIgnoreCollisions() == false && data->object->GetIgnoreCollisions() == false )
        {
            hitObject->CollisionOccurred( data->object, data->frameStamp );
            data->object->CollisionOccurred( hitObject, data->frameStamp );
        }

        return;
    }

    // Create a plane that wil act as the sliding plane.
    D3DXVECTOR3 slidePlaneOrigin = data->intersection;
    D3DXVECTOR3 slidePlaneNormal;
    D3DXVec3Normalize( &slidePlaneNormal, &( newTranslation - data->intersection ) );
    D3DXPLANE slidingPlane;
    D3DXPlaneFromPointNormal( &slidingPlane, &slidePlaneOrigin, &slidePlaneNormal );

    // Calculate the new destination accouting for sliding.
    D3DXVECTOR3 newDestination = destination - slidePlaneNormal * ( D3DXVec3Dot( &destination, &slidePlaneNormal ) + slidingPlane.d );
    newDestination += slidePlaneNormal * epsilon;

    // Calculate the new velocity which is the vector of the slide.
    D3DXVECTOR3 newVelocity = newDestination - data->intersection;

    // Check if the new velocity is too short.
    if( D3DXVec3Length( &newVelocity ) <= epsilon )
    {
        // Since the velocity is too short, there is no need to continue
        // performing collision detection. So just set the new translation
        // and velocity, then return.
        data->translation = newTranslation;
        data->velocity = D3DXVECTOR3( 0.0f, 0.0f, 0.0f );

        return;
    }

    // Set the new translation and velocity.
    data->translation = newTranslation;
    data->velocity = newVelocity;

    // Perform another collision detection recurison if allowed.
    recursion--;
    if( recursion > 0 )
        CollideWithScene( data, vertices, faces, totalFaces, objects, recursion );
}

//-----------------------------------------------------------------------------
// Entry point for collision detection and response.
//-----------------------------------------------------------------------------
inline void PerformCollisionDetection( CollisionData *data, Vertex *vertices, SceneFace *faces, unsigned long totalFaces, LinkedList< SceneObject > *dynamicObjects )
{
    // Calculate the object's translation in ellipsoid space.
    data->translation.x = data->object->GetTranslation().x / data->object->GetEllipsoidRadius().x;
    data->translation.y = data->object->GetTranslation().y / data->object->GetEllipsoidRadius().y;
    data->translation.z = data->object->GetTranslation().z / data->object->GetEllipsoidRadius().z;

    // Calculate the object's velocity in ellipsoid space.
    data->velocity = data->object->GetVelocity() * data->elapsed;
    data->velocity.x /= data->object->GetEllipsoidRadius().x;
    data->velocity.y /= data->object->GetEllipsoidRadius().y;
    data->velocity.z /= data->object->GetEllipsoidRadius().z;

    // Begin the recursive collision detection.
    CollideWithScene( data, vertices, faces, totalFaces, dynamicObjects );

    // Now set the velocity to the gravity vector (in ellipsoid space).
    data->velocity.x = data->gravity.x / data->object->GetEllipsoidRadius().x;
    data->velocity.y = data->gravity.y / data->object->GetEllipsoidRadius().y;
    data->velocity.z = data->gravity.z / data->object->GetEllipsoidRadius().z;

    // Perform another recursive collision detection to apply gravity.
    CollideWithScene( data, vertices, faces, totalFaces, dynamicObjects );

    // Convert the object's new translation back out of ellipsoid space.
    data->translation.x = data->translation.x * data->object->GetEllipsoidRadius().x;
    data->translation.y = data->translation.y * data->object->GetEllipsoidRadius().y;
    data->translation.z = data->translation.z * data->object->GetEllipsoidRadius().z;

    // Go through all the faces in the scene, checking for intersection.
    float hitDistance = -1.0f;
    for( unsigned long f = 0; f < totalFaces; f++ )
    {
        // Skip this face if its material is set to ignore rays.
        if( faces[f].renderCache->GetMaterial()->GetIgnoreRay() == true )
            continue;

        // Preform a ray intersection test to see if this face is under the object.
        float distance;
        if( D3DXIntersectTri( (D3DXVECTOR3*)&vertices[faces[f].vertex0], (D3DXVECTOR3*)&vertices[faces[f].vertex1], (D3DXVECTOR3*)&vertices[faces[f].vertex2], &data->translation, &D3DXVECTOR3( 0.0f, -1.0f, 0.0f ), NULL, NULL, &distance ) == TRUE )
            if( distance < hitDistance || hitDistance == -1.0f )
                hitDistance = distance;
    }

    // If the distance to the ray intersection is less than the radius
    // of the object along the y axis, then the object is embedded in
    // the ground. So just push the object up out of the ground.
    if( hitDistance < data->object->GetEllipsoidRadius().y )
        data->translation.y += data->object->GetEllipsoidRadius().y - hitDistance;

    // Check if the object is touching the ground.
    if( hitDistance < data->object->GetEllipsoidRadius().y + 0.1f / data->scale )
        data->object->SetTouchingGroundFlag( true );
    else
        data->object->SetTouchingGroundFlag( false );

    // Update the object's translation after collision detection.
    data->object->SetTranslation( data->translation );
}

#endif